Detection of paster welds with acoustic emission

ABSTRACT

Method and apparatus for the nondestructive detection of paster welds wherein the test piece is repeatedly flexed at the weld to be tested, and then any acoustic emission produced by such flexing, indicative of the presence of a paster weld, is detected. The invention provides for apparatus to convert the acoustic emission to electrical pulses which may then be integrated to provide a signal dependent upon the amplitude and rate of such acoustic emission.

United States Patent [191 OConnor et al.

Jan. 1, i974 OTHER PUBLICATIONS Dunegan, H. L. et al. Acoustic Emission.From Research/Development, May 1971. pp. 20-24.

Frederick, J. R. Acoustic Emission as a Technique for NondestructiveTesting. From Materials Evaluation. pp. 43-47, Feb. 70, Vol. XXVIII, No.2.

Primary Examiner.lerry W. Myracle Att0rneyCarlton Hill et a].

Method and apparatus for the nondestructive detection of paster weldswherein the test piece is repeatedly flexed at the weld to be tested,and then any acoustic emission produced by such flexing, indicative ofthe presence of a paster weld, is detected. The invention provides forapparatus to convert the acoustic emission to electrical pulses whichmay then be integrated to provide a signal dependent upon the amplitudeand rate of such acoustic emission.

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DETECTION OF FASTER WELDS WHTH ACOUSTIC EMKSSliON BACKGROUND OF THEINVENTION 1. Field of the Invention This invention is in the field ofnondestructive testing of welded objects utilizing the generation ofacoustic emission upon flexure of the welded joint, the amplitude andthe rate of the pulses emitted being indicative of the presence of adefective paster weld.

2. Description of the Prior Art In the past, there have been numeroustest procedures developed, in addition to the conventional visualexamination, for the non-destructive testing of the integrity of welds.in the case of tanks or other containers and pressure vessels, thesoundness of the welds has been tested by the application of internalpressure. The vessel is completely filled with water, and all airbubbles are allowed to escape to eliminate air pockets. After theoutlets to the vessel have been closed, a pump is operated to build upthe pressure to several thousand pounds per square inch. For pressurevessels, this test is sometimes supplemented by means ofa hammer testwhile the vessel is under a pressure of twice the determined operatingpressure. Hammer blows are struck at intervals on both sides of the weldfor a full length of the seam, followed by a complete visual inspection.

in the case of pipelines for oil or gas, an air pressure test has beenused in which a test pressure of air is built up in the system and theneach weld is painted with soapy water and inspected for bubbles whichwould indicate leaks.

Still another method which has been used in the past for detectingsoundness of welds involves determining the sound a weld gives off whentapped with a hammer. The sound weld metal provides a ringing note,whereas a faulty weld gives a flat note. A physicians stethoscope can beused to magnify and identify the sounds produced.

X-ray testing for internal defects of welds has also been employed. Manytypes of cracks, slag inclusion, blowholes, lack of fusion and otherdefects can be detected by this method. The method consists in placingan X-ray tube on one side of the piece to be tested, and a sensitivephotographic plate on the other. The resulting radiograph is a shadowpicture of material more or less transparent to radiation. The Xraysdarken the film so that regions of lower density which permitpropagation appear dark on the negative in comparison with regions ofhigh density which absorb more of the radiation. Thus, the imperfectionswhich are less dense than the base metal show up as darkened regions onthe radiograph.

A test similar to X-ray testing is performed using gamma rays. Theserays penetrate metals more rapidly than does X-ray. It is therefore moreapplicable to heavy sections which would require extremely long exposureto X-rays.

Another highly developed test for determining defects in welds is themagnetic test. The magnetic reluctance of a weld of ferromagneticmaterial is increased by any faults present in it. If a magnetic flux ispassed through the weld and the adjacent base metal, with the lines offlux approximately at right angles to the weld, there will be moreleakage flux directly over the fault than over sound sections of theweld. The faults can be detected either by sifting iron filings or ironpowder on a piece of paper placed on the welds, and observing thepicture formed or by exploring with an instrument capable of determiningthe strength of the leakage flux.

Surface imperfections in welded joints can also be located by means ofdye penetrant tests. in this type of test, an oily penetrant is appliedover the area to be tested, and allowed to dwell on the surface for ashort time. Excess penetrant is then removed, leaving the dyed penetrantin any surface discontinuities which may exist. The indications can bedeveloped by means of a dry powder applied over the test area or by theuse of a liquid suspension of such powder. The powder acts by capillaryaction to draw out any trapped dye penetrant to the surface which canthen be inspected by means of visible light or ultraviolet light,depending on the nature of the dye in the penetrant.

More recently, ultrasonics have been used for the testing of welds in aprocess involving placing a quartz crystal transmitter in intimatecontact with the metal object to be tested, by using a film of oilbetween the crystal and the metal. A short pulse of very high frequencysound is sent out by the transmitter and a reflected pulse is picked upby a crystal receiver. The output of the receiver is amplified andtranslated into a visual record. From the dimensions and geometry of thepiece under test and the velocity of sound in the material, and thepattern produced, it is possible to determine the presence or absence offlaws.

Each of the foregoing test procedures can be used to advantage indetecting defects in welds but by and large they are limited in theirapplicability to the location of defects which might be termed grossdefects. There is one type of defect, known in the trade as a pasterweld, which cannot be reliably detected by any of the foregoing methods.In this type of weld, the weld metal may make close contact with theparent metal but does not actually intermingle at the interface of thetwo metals. This lack of fusion can sometimes be detected by radiography provided that the interface is sufficiently well defined. Thereare many instances, however, where the molten metal flowing upon thecold metal may develop a partial bond or where two molten edges comingtogether may form an apparently sound union, but where the separation ofthe molten fronts still exists so that there is no true molecularmingling at the interface. Joints of this type have very littlestrength. If the beam of radiation in radiography is not parallel withthe plane of the fusion line, it will not be discovered and if the planeof fusion is extremely tight, it may also escape detection.

At one time, it was thought that the use of ultrasound would be aneffective method of detecting paster welds, but it was soon learned thatthe acoustic energy is transmitted across such barriers if the barriersare of very small dimensions, on the order of 20 to 50 Angstrom units,the dimension on the order of a molecular diameter. Consequently, awelded tube might pass an ultrasonic test and still not be as strong asa properly welded joint should be. Paster welds have been troubling thetubing industry for years and may give rise to catastrophic gastransmission line failures in the field and forming failures whenfinished tubing is formed in bending machines.

The need still remains therefore for providing methods and apparatus fordetecting paster welds reliably. The satisfaction of that need is theprincipal object of the present invention.

SUMMARY OF THE INVENTION We have now discovered that the existence ofpaster welds can be detected by straining the test piece in the area ofthe weld under conditions of plastic deformation. When a paster weld isstrained, it emits noises of an extremely low energy level over a widefrequency spectrum. A sound weld reacts to stress by plastic deformationwithout involving significant amounts of acoustic sounds. We make use ofthis characteristic by repeatedly flexing the weld joint and detectingany acoustic emission which may result upon the application of suchstress. The acoustic energy thus picked up is translated into electricalimpulses which can be amplified and used to trigger electronic circuitryto give an indication of both the pulse amplitude and repetition rate ofthe bursts of acoustic emission generated by defective welds. Thecircuitry is designed such that the significant bursts of acousticenergy can be distinguished from background noise which would tend tootherwise obscure such indications.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects, features and advantagesof the invention will be readily apparent from the following descriptionof certain preferred embodiments thereof, taken in conjunction with theaccompanying drawings, although variations and modifications may beeffected without departing from the spirit and scope of the novelconcepts of the disclosure, and in which:

FIG. 1 is partially a schematic representation and partially a blockdiagram of an overall system for detecting defective paster welds inaccordance with the present invention;

FIG. 2 is a circuit diagram of a suitable ratemeter which can be used;

FIG. 3 is a circuit diagram ofa suitable peak detector which can beemployed;

FIG. 4 is a typical chart representation obtained from a good weld; and

FIGS. 5 and 6 illustrate the type of indications obtained from pasterwelds.

-DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, there isillustrated a system for detecting paster welds in ground electrodeswhich have been welded to a spark plug shell, but it will be understoodthat the method and apparatus can be applied to a wide variety ofdevices and different types of welds.

In FIG. 1, reference numeral 10 indicates generally a spark plug shellto which a ground electrode 11 has been welded, the weld joint beingillustrated at reference numeral 12. The shell 10 is held between a pairof reciprocable clamping means 13 and 14 on opposed sides thereof, andthe ground electrode 11 is held in a clamping device identified atreference numeral 15. It should be recognized that the shell 10 can beheld in rigid relationship and the ground electrode 11 flexed to securethe same result. It is important that the ground electrode 11 be clampedin such a manner as to apply the maximum stress to the weld. Thereciprocation of the clamping means 13 and 14 through several repetitivecycles serves to induce the formation of acoustic emission if a pasterweld is present. It is believed that the noise from the paster weld isprobably due to propagation of microscopic cracks. The brittle nature ofthe bad weld enhances the amount and rate of acoustic emission.

Generally, it is advisable to limit the amount of flexure at the weldjoint such that it does not exceed an angle of about l0 from either sideof the original axis of the electrode 11.

A transducer 16 is acoustically coupled to the clamping device 15 todetect a burst of acoustic energy. Transducers suitable for this purposeare available commercially and usually include disks of doped bariumtitanate where the transducer elements normally have their rear surfacesexposed to the air, and are provided with a wear plate of a durableplastic material at their front faces. Coupling of the transducer to theclamping device may be accomplished by interposing a thin layer ofgrease between the two. The resonance frequency of the transducer mayvary widely on the order of to 300 kilo Hertz, but most acousticemission detectors operate at resonance frequencies of about kilo Hertz.

The transducer 16 converts the bursts of acoustic energy into electricalenergy pulses which are then passed to one or more amplifier stages 17which are preferably of the narrow band type and tuned to resonate atthe same frequency as the resonant frequency of the transducer 16. Anarrow band tuned amplifier is preferred because this is the mostconvenient way to achieve the required high gain and low noise requiredto permit the small acoustic emission signals to be detected. While suchnarrow band width amplifiers may tend to obscure the true waveform ofthe acoustic emission signals, this distortion is not serious becausethe rate and the amplitude of the signals are more important than theshape of the pulses.

The output of the amplifier stages 17 is used to trig ger a monostablemultivibrator 18 to generate pulses having a duration of one millisecondor so when the input level to the multivibrator 18 is above a thresholdvalue. The threshold, of course, can be adjusted by a sensitivitycontrol in the multivibrator circuit.

Pulses generated by the multivibrator 18 are then passed to a ratemeter19 which senses the rate at which the multivibrator is being triggered.A suitable circuit for such a ratemeter is shown in FIG. 2, and includesa coupling capacitor 20 which delivers the signal between a pair ofdiodes 21 and 22. A pair of capacitors 23 and 24 is connected inparallel and a DC. potential whose amplitude is proportional to thepulse rate appears across a resistor 25 in parallel with the capacitors23 and 24.

The output of the ratemeter circuit 19 is passed to a peak detector 26which may consist, as shown in FIG. 3, of a diode 27 and a capacitor 28.As illustrated in FIG. 1, the output of the peak detector 26 mayoptionally be passed to alarm circuitry 29 which energizes a visual oraudible alarm at a sufficiently high input intensity, or to a recorder30 such as a cathode ray tube or a strip chart recorder which provides apermanent record of the pulses.

In FIG. 4, there is a representation of the type of pattern observed ona strip recorder when testing a sound weld in accordance with theprocedures of the present invention. The horizontal axis is the timeaxis, and the vertical axis represents amplitude. As illustrated in thatfigure, only a few low amplitude pulses are observed when testing asound weld. This test was confirmed by a destructive test where it wasfound that it required thirty bends to produce failure, the failureoccurring in the wire rather than at the weld.

The results obtained from the chart of FIG. 4 should be compared withthose of FIGS. 5 and 6. Both these charts represent results obtainedfrom acoustic emission tests of defective paster welds, plotted on thesame time and amplitude axis as that of FIG. 4. It will be noted thatthe amplitude and pulse repetition frequency of the pulses in both thesecharts are significantly higher than that in FIG. 4. Mechanical testingof both these welds showed failure of the welds by failure at six toseven cycles of bending.

It should be evident that various modifications can be made to thedescribed embodiments without departing from the scope of the presentinvention.

We claim as our invention: 7

l. The method of testing a test piece for a paster weld which comprisessupporting said test piece rigidly at an area spaced from the weld to betested and along a predetermined axis, oscillating the test piecerepeatedly while so supported to thereby cause flexing of said weld toboth sides of said predetermined axis and detecting any acousticemission produced by such flexing.

2. The method of claim 1 in which said flexing is applied to cause adeflection of said test piece of not more than about 10 from itsoriginal axis.

3. The method of claim 1 which includes the step of converting saidacoustic emission into electrical pulses and integrating said electricalpulses to provide a signal dependent on the amplitude and rate of suchacoustic emission.

4. The method of claim 3 which includes the step of operating an alarmdevice when said signal reaches a predetermined level.

5. An apparatus for detecting a paster weld which comprises means forholding the device to be tested in clamped engagement along apredetermined axis, means for repeatedly flexing said device at the weldto be tested on both sides of said axis, and sensor means positioned topick up any acoustic emission resulting from such flexing.

6. The apparatus of claim 5 which includes an integrating means arrangedto provide a signal dependent upon the size and rate of any acousticemission picked up by said sensor means.

7. The apparatus of claim 5 which includes electrical pulse generatingmeans actuated by electrical signals from said sensor means and ratedetecting means responsive to the rate of pulses received from saidpulse generating means.

1. The method of testing a test piece for a paster weld which comprisessupporting said test piece rigidly at an area spaced from the weld to betested and along a predetermined axis, oscillating the test piecerepeatedly while so supported to thereby cause flexing of said weld toboth sides of said predetermined axis and detecting any acousticemission produced by such flexing.
 2. The method of claim 1 in whichsaid flexing is applied to cause a defLection of said test piece of notmore than about 10* from its original axis.
 3. The method of claim 1which includes the step of converting said acoustic emission intoelectrical pulses and integrating said electrical pulses to provide asignal dependent on the amplitude and rate of such acoustic emission. 4.The method of claim 3 which includes the step of operating an alarmdevice when said signal reaches a predetermined level.
 5. An apparatusfor detecting a paster weld which comprises means for holding the deviceto be tested in clamped engagement along a predetermined axis, means forrepeatedly flexing said device at the weld to be tested on both sides ofsaid axis, and sensor means positioned to pick up any acoustic emissionresulting from such flexing.
 6. The apparatus of claim 5 which includesan integrating means arranged to provide a signal dependent upon thesize and rate of any acoustic emission picked up by said sensor means.7. The apparatus of claim 5 which includes electrical pulse generatingmeans actuated by electrical signals from said sensor means and ratedetecting means responsive to the rate of pulses received from saidpulse generating means.